Cavity slot antenna



543-872 XR 3550141 5R Dec. 22, 1970 J, HARRIS ETAL 3,550,141

CAVITY SLOT ANTENNA Filed Feb. 5, 1969 2 Sheets-Sheet l VSWR=3.0=I 3I800 mc BANDWIDTH VSWR=2.0=| 2 I I600 me 7 l BANDWIDTH o 1 VSWR o 1 o o1 o 1 I -O--o-o 1 E l i 1 i i 1 l I N 1 j i 1 4 4 i I I 0 h i I i 405000' 5500 6000 6500 7000 FREQU ENCY (MEGACYCLES) John R. Harris John W.Tse Gerald H. Hooper INVENTORS %.o.| MM BY WWWM AGENT DQC. 22, 1970HARRls ETAL 3,550,141

CAVITY SLOT ANTENNA Filed Feb. 5, 1969 2 Sheets-Sheet 2 States PatentUS. Cl. 345-461 Claims ABSTRACT OF THE DISCLOSURE An antenna in which apair of diametrically disposed and interconnected kidney-shaped slots inone end of a substantially] cylindrical housing forms a resonant cavitytherein. dielectric window is secured to the end of the housing to coverthe open cavity.

BACKGROUND OF THE INVENTION This invention generally relates to antennastructures and more: particularly to antennas of the cavity slotvariety.

Heretofore, antennas of the wave guide, single slot, multiple slot, andwire or rod-types have been commonly used. wh re; limited space has beena prime factor, however, the p'revious antenna-s have been tunedinivan'ous ways, which correspondingly have limited their frequencybandwidths and gains. Among the most suitable antennas of the prior artare the flush-mounted, cavity slot types, wherein the. cavity design hasbeen based on a foaming technique." The electrical properties of thepolyurethane foam, however, are very sensitive to change in density andchemicalcomposition of the foam. It is evident therefore that differentmixing and curing techniques and the size of the container to be foamedall affect the density, and thereby the electrical properties of 'thefoam.

Such has" resulted in a high rejection rate of antennas of this type.

One of the major problems in the design of antennas adapted for use inaircraft or missiles is to meet the thermal reguirements. Due to thedifferent thermal conductivitiesi'a'the heat flow problems are quitecomplex. For example, the external temperature of the cavitycould reacha very high temperature, and the performance of the antenna might bedetuned if materials of low thermal conductivity areused. The internalportions of the antenna will of course be raised to very hightemperatures,

regardless of the materials used.

. SUMMARY OF THE INVENTION It is, therefore, an object of this inventionto provide an antenna having a substantially constant impedance over awide frequency bandwith and a broadbeam radiation pattern. 3

Another object; of the present invention is the provision of a small andcompact antennasuitable for flush mount installation on an aircraft ormissile.

Yet another object of this'invention is the provision of an antennacomprised of a minimum number of parts and having a low manufacturingcost.

Still another object of this invention is to provide an antenna havinghigh resistance to high voltage breakdown at very high altitudes.

Briefly, in accordance with our embodiment of the invention, theforegoing and other objects are attained by an antenna made up of onlyfive basic parts and being suitably designed for flush mountinstallation. The parts of the antenna include a housing forming thecavity, a

- dielectric window covering the cavity, a retaining lock ring, amounting flange and an RF connector.

many of its attendant advantages will be readily apparent as the samebecomes better understood by reference to the detailed description whenconsidered in connection with the accompanying drawings wherein:

FIG. 1 is a side cross-sectional view of one embodiment of thisinvention;

FIG. 2 is a cross-sectional view of the device taken along the line 2-2of FIG. 1, showing internal construction of the antenna and particularlythe novel shape" of the cavity formed therein;

FIG. 3 is a cross-sectional view of the device taken along the line 3-3of FIG. 2;

FIG. 4 is a graphical illustration showing the variation of VSWR(voltage standing wave ratio) with frequency for a cavity antenna of thepresent invention; and

FIG. 5 is a graphic presentation of a typical radiation pattern for anantenna of the present invention taken for a roll plane cut.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now in greater detailto FIGS. 1 to 3 of the drawings, the antenna structure of the inventioncomprises a substantially cylindrical-shaped housing 10 of anelectrically conductive metal, having formed therein a cavity made up ofa pair of diametrically opposed kidneyshaped slots 12 and aninterconnecting linear slot 14. An

axial bore 16 through housing 10 is positioned adjacent one side of thelinear slot 14 and between adjacent ends of the kidney-shaped slots 12.At the end of the housing- 10 wherein the .cavity is formed, a slot 18of the same width as the diameter of bore 16 and of less depth, connectsthe bore with the linear slot 14 and thereacross to certain super alloysof steel, or other metals exhibiting very high thermal conductivities. 7

It should be readily apparent that the depth of the cavity formed inhousing 10 may be varied in accordance with the requirements of aparticular application and frequency response desired.

A coaxial 50 ohm RF transmission line 22, having an inner conductor 24,is passed through the bore 16 in the housing 10 and is securely fastenedthere to be an RF connector 26. The inner conductor 24 is bent over theslot 18 and across the linear slot 14 of the cavity, and is press fittedor soldered into the narrow channel or receiving aperture 20.

A dielectric cover plate or window 28 is secured to the open end ofhousing 10, thereby protecting the cavity from intrusion of moisture orother foreign matter which could affect the tuning of the antenna. Asmore shown in FIG. 1 of the drawings, the window 28 is substantiallydisc-shaped in configuration to fit matingly against the end of housing10 and is provided with an end face 30 of reduced diameter adapted toengage a flange 32 of a mounting shell 34. The shell 34 is exteriorlythreaded at the flanged end thereof for attachment to the aircraft ormissile and is interiorly threaded at the other end for engagement with.a lock ring 36. An end of lock ring 36 abuts a shoulder 38 formed onthe circumferential wall of housing 10 when it is threaded into themounting shell 34, and thereby secures the five basic elements of theantenna together.

The particular arrangement of the antenna cavity and Patented Dec. 22,1970 H i I g 1. 1. l .1-2tilii. t-t'isiattm. feed lends itself topotting the cavity housing and .the window 28 simultaneously in a verysimple fixture.

One approach found to be acceptable is using eccoceram, a ceramicpotting compound of which there are two types. Their dielectricconstants range from 2.8 to 3.6. After potting, this material willsustain continuous temperaturesto 2500 F.

Although many materials may be acceptable, the material from which thewindow 28 is constructed must be able to handle very high temperatures,such as those encountered during re-entry into our atmosphere of certainmissiles. One such material is boron nitride.

If desired,the antenna may be hermetically sealed at atmosphericpressure to reduce high voltage breakdown at high altitude.

FIG. 4 is a typical data plotting for the embodiment of FIGS. 1-3,wherein voltage standing wave ratio (VSWR) is plotted as a function offrequency of the electromagnetic wave signal being radiated bytheantenna. It is readily evident from this characteristic curve that thefrequency bandwidth and the impedance factor obtained with the antennaof this invention is much improved over that of conventional cavityantennas. Exemplary of this is the fact that a bandwidth of 1800megacycles below a VSWR of 3.0 to 1 is achieved with the novelkidneyshaped cavity antenna of this invention.

Likewise, FIG. 5 is a graphic showing of the radiation pattern for theembodiment of FIGS. 1-3 taken for a roll plane cut. Here it may beobserved that the antenna gain is 3.0 db above an isotropic radiator,and the beamwidth is 110 at the --3.() bd points.

Briefly stated in summary, according to the present invention, a compactcavity antenna designed for flush mount installation is provided havinga substantially constant impedance over a wide frequency bandwidthand abroadbeam radiation pattern.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced 2. An antenna according to claim ll wherein saidhousing is constructed of beryllium.

3. An antenna according to claim 1 wherein said cover plate isconstructed of boron nitride.

4. An antenna according to claim 1 wherein said energy introducing meansis a transmission line extending into said housing between saidkidney-shaped slots and adjacent said linear slot,

said line being bentover said linear slot and secured to said housing. 1I

5. A wideband antenna structure according to claim 1 wherein saidhousing is substantially cylindrical-shaped and constructed'ofberyllium, W

said cover plate is substantially disc-shaped and constructed of boronnitride, and

said energy introducing means is a transmission line extending into saidhousing from the end opposite said cover plate between saidkidney-shaped slots and adjacent said linear slot,

said line being bent over said linear slot and secured on the oppositeside thereof to the housing.

6. Ah antenna according to claim 1 wherein said housing and said coverplate are secured with a ceramic potting compound.

7. An antenna according to claim 1 wherein said cavity is hermeticallysealed at atmospheric pressure.

8. An antenna according to claim 5 wherein said housing and saidcoverplate are secured with a ceramic potting compound, and

said cavity is hermetically sealed.

9. .An antenna according to claim 5 wherein said housing and said coverplate are connected by a tubular shell and a lock ring,

said lock ring and said shell enclosing said housing 'and said coverplate and being threadably secured to each other.

10. An antenna according to claim 9 wherein said housing and said coverplate are further secured with a ceramic potting compound, and

said cavity is hermetically sealed.

. References Cited UNITED STATES PATENTS 3,262,119 7/1966 Sisson 343771XELI LIEBERMAN, Primary Examiner M. NUSSBAUM, Assistant Examiner us. 01.X.R.

